cython wrapper for khash-sets/maps, efficient implementation of isin and unique

 Cykhash is a cython wrapper for khash-sets/maps, efficient
 implementation of isin and unique.
 .
   * Brings functionality of khash to Python and Cython and can be used
     seamlessly in numpy or pandas.
   * Numpy's world is lacking the concept of a (hash-)set. This
     shortcoming is fixed and efficient (memory- and speedwise compared
     to pandas') unique and isin are implemented.
   * Python-set/dict have big memory-footprint. For some datatypes the
     overhead can be reduced by using khash by factor 4-8.